Spectrometric instruments include a dispersion element, such as a diffraction grating, and a detector system. Modern instruments include a computer that is receptive of spectral data from the detector to analyze and compare spectra. With improvements in optics, detectors and computerization, there has evolved an ability to perform very precise measurements.
One type of instrument utilizes an inductively coupled plasma (ICP) with sample injection to effect spectral lines of atomic elements. A spectrophotometer used in conjunction with ICP is a crossed grating type that produces a two dimensional array of spectral lines. A detector for the array has segmented subarrays of photosensitive pixels located strategically only at expected locations of the spectral lines. A solid state device such as a charge coupled device (CCD) with the photosensitive pixels in the subarrays is used. A crossed grating spectrophotometer incorporating such a detector is disclosed in U.S. Pat. No. 4,820,048 (Barnard).
With the evolving requirements for precision, variations among instruments, and drift in each instrument (e.g. from temperature and pressure variations) have become more of a problem. U.S. Pat. No. 5,303,165 (Ganz et al, "Ganz patent") discloses standardization of instruments by transforming spectral data with a transformation filter derived from a specified line profile common to the instruments. Spectral lines as displayed by a spectrometric instrument actually have a finite width and profile, and the standardization corrects for variations in instrument profile. Such standardization is distinguished from calibration associated with quantification. Determination of compositional quantities of a sample is carried out separately or in conjunction with standardization. Wavelength calibration may be associated with standardization, but actual wavelengths of measured spectra are not necessarily needed. The technique of the Ganz patent is quite precise and useful, particularly with continuous array detectors, but suffers from a requirement for substantial amounts of spectral data collection and associated lengthy computations. The Ganz patent also discloses the use of a source of regular fringe peaks for wavelength calibration, but such use is not suited for a segmented subarray detector.
The presence and quantity of components in a sample may be determined with computer computations by application of calibration models to spectral data, the models being derived from spectra of known quantities of sample analytes (individual atomic elements). An archive of model data is stored in computer memory for application to sample data for essentially automatic determination of components and their quantities in the sample. One approach is disclosed in U.S. Pat. No. 5,308,982 (Ivaldi et al) which incorporates a derivative of sample spectral data into the matrix model to compensate for spectral drift. This is a standardization that requires spectral data to be acquired in relatively small spectral increments to achieve sufficient representation of the derivative in the model. Wavelength increments of spectral data ordinarily is limited by pixel size. Smaller increments are achieved by slit scanning in which the inlet slit to the spectrometer is imaged on a pixel. Varying the lateral position of the slit in small steps effectively moves a spectrum across the pixels to obtain spectral data in smaller increments. Although utilized for collecting archive data, it is preferable that slit scanning be avoided to speed up ordinary data acquisition.